The CIP, proof authority for members states that include most major shotgun manufacturing countries, state clearly that 67mm shells may be fired in 65mm chambers. But specifically warn against using other long shells in short chambers. Sixty seven millimeters is 2 5/8 inches according to my calculations.
We should be cautious with opinions on this subject considering that some not so knowledgeable readers read these posts. The safe practice, according to CIP is not to use 70mm (2 3/4) in shorter chambers. Same goes for 3 inch and so on.
